In recent years, various vehicular navigation systems capable of informing the driver of the current location of the vehicle have been developed. One known navigation system of this type detects the distance traveled and the relative travel direction of the vehicle at prescribed time intervals, successively calculates the vector sum of these detection results, and displays the current location of the vehicle by means of an appropriate display device on the basis of the calculated result and the given initial vehicle location information.
This kind of navigation system requires detection of the relative direction of the vehicle as described above, and for this reason there is a well-known arrangement for detecting the relative travel direction on the basis of the difference between the rotational angle of the left wheel and that of the right wheel, which occurs at the time of vehicle turning, by means of rotational sensors on a pair of wheels.
Japanese Patent Application Public Disclosure No. Sho 62-298716, for example, discloses a method for detecting the relative travel direction of a vehicle, in which information concerning rotational angles of the left and right front wheels is obtained by means of rotational speed sensors mounted on left and right front wheels, information concerning the average distance traveled by the rear wheels is also obtained, and on the basis of this information, information concerning the relative travel direction is obtained free from the detection error owing to the steering mechanism.
However, this proposed method has a disadvantage in that since at least one sensor is needed for detecting the average distance traveled by the rear wheels in addition to two speed sensors for detecting the wheel rotation angles of the two front wheels, the number of sensors required is large and the processing circuit for processing the signals generated by the sensors is complicated.
To overcome these disadvantages, an arrangement making use of the rotation of the cable of the already installed speedometer can be used. According to this method, although the number of sensors is not increased, the accuracy cannot help being unsatisfactory, so that it is difficult to detect the location of the vehicle with high accuracy.
It is an object of the present invention to provide an improved relative direction detecting method for a vehicle which is capable of detecting the relative travel direction of the vehicle with high accuracy only by detecting information concerning the rotational angles of the left and right front wheels.